The oxidation of silicon is a key process in both MOS and bipolar technology since oxides are widely used for gate insulators, oxide masks, and thick field and isolation regions.
A common method of oxidizing silicon is high temperature oxidation However, such high temperature oxidations affect junction motion, change impurity profiles, generate stacking faults and dislocations, and may also increase the possibility of oxide contamination. All of the foregoing effects are considered undesirable, particularly when such processes affect circuitry adhering to one micron design rules.
Several processes have been explored in order to reduce the negative effects of high temperature oxidation. One obvious approach is the reduction of oxidation temperature using such techniques as anodic oxidation (A. K. Vijh, Oxide and Oxide Films, Vol. 2, J. W. Diggles, Marcel Dekker, 1963, p. 46), anodic oxidation in an oxygen plasma (J. R. Ligenza, J. Appl. Phys., 36, #9, 2703 (1965)), and pressurized oxidation (L. Katz and L. D. Kimerling, Electrochem Soc., 125, 1680 (1968)). In addition, low temperature chemical deposition techniques (CVD) for production of oxide films have also been explored. In general, however, the electrical quality of deposited oxides is generally unsatisfactory unless a high temperature anneal is employed.
A more successful approach involves the use of low level chemical additives during a standard high temperature oxidation period. In particular, the most common approach involves standard 1 atmosphere dry or steam oxidation with the addition of HCl, Cl.sub.2, or TCE (C.sub.2 HCl.sub.3) to the oxidizing ambient. Originally, these chlorine compounds were introduced as a vapor "getter" to reduce mobile ion contamination within the oxidation furnace. (S. Mayo and W. H. Evans, Electrochem. Soc., 124, 780 (1977)). The use of chlorine additives to the oxidation ambient was particularly successful and has been the topic for many investigations. (R. J. Kriegler, Y. Cheng, D. R. Colton, J. Electrochem. Soc., 119, 388 (1972); R. J. Kriegler, Semiconductor Silicon 1973. ed. Huff and R. Burgess, Electrochem. Soc., p. 363 (1973); R. J. Kriegler, Thin Solid Films, 13, 11 (1972))
In addition to the passivation of ionic sodium, addition of chlorine to the oxidation ambient has been reported to improve the lifetime of the underlaying silicon (D. R. Young and C. M. Osburn, J. Electrochem. Soc., 120, 1578 (1973)) and also results in improved oxide breakdown strength. (C. M. Osburn, J. Electrochem Soc., 129, 809 (1974)). Because of the importance of the chlorine enhanced oxidation process in MOS technology, it has been thoroughly studied. The relationship between chlorine content and sodium ion passivation has been investigated (A. Rohatgi, S. R. Butler, and F. J. Feigl, J. Electrochem. Soc., 126, 149 (1979)). Furthermore, it has been noted that the addition of chlorine to the oxidation ambient increased the oxidation rate under dry conditions. Specifically, rate enhancements of about 30% for up to 10% added HCl and approximately 60% for 2.5% added Cl.sub.2 (over the rate for standard dry O.sub.2).
It is known that the addition of chlorine to the oxidation ambient has many adverse effects. Particularly, small amounts of water causes a loss of effective partial pressure of chlorine in the system. (R. J. Kriegler and Denki Kogaku, 41, 466 (1971); K. Ehara, K. Sakmara, and K. Ohwada, Elec. Soc., 120, 526 (1973)) Also, in commonly employed chlorine concentrations, a chlorine rich phase has been shown to develop at the Si/SiO.sub.2 interface. This process eventually degrades oxide adhesion (J. Monkowsi, et al, J. Electrochem. Soc., 125, 1867 (1978); S. L. Titcomb and F. J. Feigl, Electrical Properties of HCl Oxides, presented at ISSC 1982 San Diego.
For the production of thin (&lt;100 .ANG.) gate insulators, silicon nitride possesses superior properties including a higher dielectric constant, superior breakdown strength, and better electrical uniformity than silicon dioxide. Prior art silicon nitride films grown by thermal nitridation techniques, however, were highly contaminated with oxygen. (Ito, et al. J. Electrochem. Soc., 125 448 (1978).)
Thicker films having higher nitrogen content have been produced in NH.sub.3 nitridation ambients instead of N.sub.2 ambients by employing elevated temperatures. These films, however, appear not to grow significantly thicker with increasing nitridation time. Finally, plasma nitridations have been practiced, with mixed results.
Finally, a number of practical problems are associated with chlorine addition to the oxidation ambient. These include the highly corrosive nature of the ambient vapors which makes heavy demands on corrosion resistance of equipment, and exhaust facilities. Furthermore, the environmental effects of exhausting such vapors can be extremely serious.
It has been observed by various investigators that the addition of fluorine ions to wet anodization baths for silicon and to oxygen or nitrogen plasmas can result in enhanced rates of oxidation. (P. F. Schmidt and W. Michel, J. Electrochem. Soc., 104, 230 (1957); M. Croset and D. Dieremegard, J. Electrochem. Soc., 120, 426 (1973); R. P. H. Chang, C. C. Chang, and S. Dorack, Appl. Phys. Lett. 36, 999 (1981))